750 SF2 Mondial pistons

Not to beat a dead horse, but while trying to determine the Ross piston wear, attached are the Ross pistons on the right that show extreme wear on the intake sides after @ 3000 miles, and the original Mondials that came w the bike at 16,000 miles. I installed .5 over Ross pistons after the worn cylinders were re-bored. I notice that the Mondials are taller, have a different skirt profile, and smaller valve depressions on the intake side. Forgive my piston ignorance, but could these differences be responsible for the wear? Would a shorter piston "rock" more on the upswing, and would a longer piston be more stable? I'm trying to decide which to use and realize Mondials are no longer available. I can't re-use the Mondials since the top ring grooves are worn past the 0.10 mm limit.
Thank you
 

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Piston height of the Ross is a non-issue (check out the 1290 KTM piston below!). Can safely say your wear has nothing to do with the piston height and 'rocking' back and forth. Have you actually measured the extent of wear on those Ross slugs - comparing the worn section with anywhere fore-aft that hasn't been 'rubbing'? I'd be interested to know just how much diameter has been worn away - could be negligible.

To compare pistons I generally look at them both attached to one piston pin. Pic shows a low comp Ross and an Asso SFC. Larger valve pocket may just be to accommodate larger inlets.

Short piston.jpg

Ross-Asso.SFC.JPG
 
I'm no expert, but aren't standard Mondial pistons cast, and Ross are forged? The latter are way stronger. Pretty sure Red gets his pistons coated to up their game even more to withstand really hard use.
 
Thanks Pirhana,
I,ve gotten different opinions about the piston wear, from incorrect re-bore clearance, to gas wash, to oil starvation. The wear on the Ross has been measured by a machinist who says it’s beyond spec, and the bore was remeasured and found to be true with correct .003” clearance.
The oil was analyzed and I was told the gas content is acceptable, it could be from the different skirt design, and possibly from using Bel Ray 20/50 mineral, and that a synthetic blend would be better.
So I’ll try the Ross w better oil and hope for the best.
 
TeddyG said:
Thanks Pirhana,
I,ve gotten different opinions about the piston wear, from incorrect re-bore clearance, to gas wash, to oil starvation. The wear on the Ross has been measured by a machinist who says it’s beyond spec, and the bore was remeasured and found to be true with correct .003” clearance.
The oil was analyzed and I was told the gas content is acceptable, it could be from the different skirt design, and possibly from using Bel Ray 20/50 mineral, and that a synthetic blend would be better.
So I’ll try the Ross w better oil and hope for the best.
Click to expand...
It has nothing to do with oil quality.
Paul
 
Did the pistons rock when fitted? By this I mean, with the head off could you rock the piston with your fingers at TDC? I recently did a motor that I believe had a new piston in an old worn bore, you could hear the piston rocking in the bore as well as seeing it move back to front.
Bel ray 20/50 is an ideal oil so it’s not that. Your piston wear is very odd and so is the carbon deposit on the top of the piston- at that mileage from a rebore it should only have a light discolouring that would wipe off- your valve pockets look almost full of carbon. Have you contacted Ross? I would send them either pics or the pistons and see what they say- they will give you a definitive answer . I wouldn’t just rebore it again without finding out why the first repair was unsuccessful .
 
TeddyG said:
Thanks Pirhana,
I,ve gotten different opinions about the piston wear, from incorrect re-bore clearance, to gas wash, to oil starvation. The wear on the Ross has been measured by a machinist who says it’s beyond spec, and the bore was remeasured and found to be true with correct .003” clearance.
The oil was analyzed and I was told the gas content is acceptable, it could be from the different skirt design, and possibly from using Bel Ray 20/50 mineral, and that a synthetic blend would be better.
So I’ll try the Ross w better oil and hope for the best.
Click to expand...
.003" seems tight for a forged piston. @redax5 will know. @TeddyG - I have a stock bore 750 barrel if you have a need, name your price!
 
Ross pistons I have in the 750 engine have 7/100 mm clearance, no issues.
I assume you sourced the pistons from a well known parts dealer who would have given you the required clearance infomation, be it imperial or metric. Have the bores been measured left to right, back to front at bottom, middle and top of each liner?
 
Thanks guys,
I bought the Ross pistons from Wolfgang who recommends .003” clearance, several folks including the oil analyst have advised me not to use mineral oil, the bores have been re- measured in several spots for trueness, the pistons were inserted correctly, and the pistons do rock due to the extreme wear. I contacted Ross several times with no response and they’re moving with a 2 month lead time. The oil test indicated the mineral oil lost viscosity, and modern synthetics/blends have a higher shear stability. Thanks for your generous offer IAG!
 
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Hi Teddy,

I would remove the primary Cover and change the Oil-Pump to a 8mm or 10mm one ( original are 6 mm ).
Count the teeth of the Oil-Pump drive-gear.
Should be 58 teeth, not 44.
If 44, change to 58
( with all Parts are needed ).
You can remove the Oil-Pump drive Gear when the Key for the auto-advance-unit is on 3 or 9 a clock. Be careful, when you remove it, dont use any Tool for Removing, only your Fingers.

Chapter 05

IGNITION, OIL PUMP , Laverda 750
www.laverda-paradies.de

Rainer
 
Hmmm - can I hear an oil thread revving up in the foyer? :cool:

Have never heard of anyone being told specifically NOT to use a mineral oil in any Laverda Breg twin or triple - usually the direct opposite in fact. I ran the 880 SFQ in (quickly) on mineral oil, standard 20/50 stuff, and now use Mobil 1 V-Twin 20/50. I love Mobil 1 - proven top quality stuff, but it's quite hard to find and I'll probably go for an alternative synth if I can guarantee the quality and it won't create clutch issues. The Mobil 1 has never caused even the slightest clutch slip and it has to deal with a big jump in HP and torque from a stock SF - and anyone who's ridden with me will confirm that i don't baby the thing - it gets a right caning.

I do hope you can get to the bottom of the problem, Teddy, and end up with a nice crisp SF2 that does all the wonderful things a good SF should do.
 
Vince said:
I'm no expert, but aren't standard Mondial pistons cast, and Ross are forged? The latter are way stronger. Pretty sure Red gets his pistons coated to up their game even more to withstand really hard use.
Click to expand...
been using Ross pistons for close on 20 years now and yes I get them ceramic coated on the crowns and anti-friction coated on the skirts
they are forged, Mondial's are cast


IAG said:
.003" seems tight for a forged piston. redax5 will know.
Click to expand...
I bore and hone barrels to 0.065-0.070mm to suit the Ross pistons including on my race engines which is slightly tighter then 0.003"
Only ever use Mineral oil on all my Laverda engines including my race engines, not interested in using Synthetic oil in them

We do run Synthetic oil though in our LS1 in my 1950 Chevy pickup, LS2 in Maxene's HSV GTO and her Maserati GT engines but stick with mineral in her Corvette small block Chevy V8
 
Thanks Redax and Rez,

Since the Ross forged expand a bit more than cast, I was wondering if .003” is a bit too tight, and I,ve considered checking the oil pump. I’ve also been advised to get the skirts Teflon coated.
Regarding oil, SpeedDiagnostics who analyzed the 1300 mile mineral oil said it had lost viscosity and has a lower shear stability, so perhaps this is due to some other factors causing this.
 
It’s a real shame what happened to you Teddy! After all that’s been written so far, I’d suspect that piston clearance and/or the oil supply are the likely culprits here.

So, the question arises: is the 0.003" piston clearance correct and actually present at multiple measuring points? If it really is a consistent 0.003" across all areas, then - according to Red - that should be fine.

As for the oil: I don’t think it has anything to do with whether the oil is mineral-based or not. Comments from various folk and the oil analyst you mentioned are just confusing you. Laverdas are old clunkers with roller-bearing crankshafts. Trust the experts here; they know what they’re recommending and why. In this context, it has often been suggested to use Harley oil specifications as a guideline.

Regarding the oil supply function, it is obviously important to determine whether oil is actually reaching the piston/barrel area. Not much gets there anyway, but it’s still worth checking to ensure an oil passage isn't blocked somewhere.

You’re chasing down a lot of different leads, and understandably, your trust in the previous expert has been completely shattered. I strongly recommend you to find a new specialist and start tackling the issue afresh with him. Of course, I realize there aren't many such experts around - especially not in your immediate area.
 
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